1. Field of the Invention
The present invention relates to a robot arm mechanism having arms contracted and extended, and more particularly to a robot arm mechanism incorporating an arm driving mechanism for driving the arms to assume its contracted and extended positions.
2. Description of the Related Art
Up until now, there have been proposed a wide variety of conventional robot arm mechanisms available in the process of producing semiconductors in which the robot arm mechanism is operated to have robot arms contracted and extended to handle works, i.e., objects to be treated. These objects may include works such as, for example, wafers and other precision parts that are to be transferred and then unloaded onto a work table by the robot arm mechanism.
One typical example of the conventional robot arm mechanism of this type is disclosed in, for example, Japanese Patent Laying-Open Publication No. 2002-222488 and shown in FIG. 16. The conventional robot arm mechanism 910 is shown in FIG. 16 as comprising a first handling member 951, a second handling member 952, a first robot arm 910a, and a second robot arm 910b. 
The first handling member 951 is operative to support and handle a first object, while the second handling member 952 is operative to support and handle a second object. The first robot arm 910a includes a first arm link 931 having a first end portion, a second end portion, a longitudinally intermediate portion intervening between the first end portion and the second end portion, and a central line passing through the first end portion and the second end portion, and a second arm link 941 having a first end portion, a second end portion, a longitudinally intermediate portion intervening between the first end portion and the second end portion, and a central line passing through the first end portion and the second end portion.
The second end portion of the first arm link 931 is pivotably connected to the first end portion of the second arm link 941, while the second end portion of the second arm link 941 is connected to the first handling member 951 to allow the first handling member 951 to support the first object in a stable condition.
The second robot arm 910b includes a first arm link 932 having a first end portion, a second end portion, a longitudinally intermediate portion intervening between the first end portion and the second end portion, and a central line passing through the first end portion and the second end portion, and a second arm link 942 having a first end portion, a second end portion, a longitudinally intermediate portion intervening between the first end portion and the second end portion, and a central line passing through the first end portion and the second end portion.
The first arm link 941 is inclined with respect to the second arm link 942 at a preset angle defined between the central line of the second arm link 941 of the first robot arm 910a and the central line of the second arm link 942 of the second robot arm 910b. The second end portion of the first arm link 932 is pivotably connected to the first end portion of the second arm link 942, while the second end portion of the second arm link 942 is connected to the first handling member 952 to allow the first handling member 952 to support the first object in a stable condition. The conventional robot arm mechanism 910 further comprises an angle keeping mechanism 960 for keeping substantially fixed the preset angle defined between the central line of the second arm link 941 of the first robot arm 910a and the central line of the second arm link 942 of the second robot arm 910b. 
The angle keeping mechanism 960 includes a first link 961, a second link 962, a third link 963, a fourth link 964, a fifth link 965, and a sixth link 966. The first link 961 has a first end portion and a second end portion. The second link 962 has a first end portion, a second end portion, and a central line passing through the first end portion and the second end portion. The first end portion of the second link 962 is pivotably connected to the longitudinally intermediate portion of the second arm link 941 of the first robot arm 910a. The second end portion of the second link 962 is pivotably connected to the first end portion of the first link 961 under the state that the central line of the second link 962 is in parallel relationship with the central line of the first arm link 931 of the first robot arm 910a. 
The third link 963 has a first end portion, a second end portion, and a central line passing through the first end portion, the second end portion, and a longitudinally intermediate portion intervening between the first end portion and the second end portion. The first end portion of the third link 963 is pivotably connected to the longitudinally intermediate portion of the second arm link 942 of the first robot arm 910b, while the second end portion of the third link 963 is pivotably connected to the second end portion of the first link 961 of under the state that the central line of the third link 963 is in parallel relationship with the central line of the first arm link 932 of the second robot arm 910b. The fourth link 964 has a first end portion and a second end portion. The first end portion of the fourth link 964 is pivotally connected to the longitudinally intermediate portion of the first arm link 931 of the first robot arm 910a. 
The fifth link 965 has a first end portion and a second end portion. The first end portion of the fifth link 965 is pivotally connected to the second end portion of the fourth link 964, while the second end portion of the fifth link 965 is pivotally connected to the longitudinally intermediate portion of the first arm link 932 of the first robot arm 910b. The sixth link 966 has a first end portion and a second end portion. The first end portion of the sixth link 966 is pivotally connected to the longitudinally intermediate portion of first arm link 932 of the second robot arm 910b, while the second end portion of the sixth link 966 is pivotally connected to the longitudinally intermediate portion of third link 963. The sixth link 966 is integrally formed with the fifth link 965 under the state that the first end portion of the sixth link 966 is connected to the second end portion of the fifth link 965. The central line of the sixth link 966 is substantially in parallel relationship with the central line of the second arm link 942 of the second robot arm 910b. 
The angle keeping mechanism 960 further includes a first driving shaft 921 connected to the first arm link 931 of the first robot arm 910a, and a second driving shaft 922 connected to the second arm link 932 of the second robot arm 910b; and a connecting shaft 923 connected to third link 961.
The conventional robot apparatus 900 comprises the conventional robot arm mechanism 910 as previously mentioned, and first and second driving motors not shown in FIG. 16. The first driving motor is operative to drive the first driving shaft 921 to have the first driving shaft 921 pivoted clockwise and counterclockwise around the pivotal axis within a pivotal movement range. The second driving motor is operative to drive the second driving shaft 922 to have the second driving shaft 922 pivoted clockwise and counterclockwise around the pivotal axis within the pivotal movement range.
The angle keeping mechanism 960 is operative to keep substantially fixed the preset angle defined between the central line of the second arm link 941 of the first robot arm 910a and the central line of the second arm link 942 of the second robot arm 910b while the first and second driving shafts 921 and 922 are respectively pivoted clockwise and counterclockwise around the pivotal axis by the first and second driving motors to have one of the first arm links 931 and 932 of the first and second robot arms 910a and 910b angularly moved with respect to the other of the first arm links 931 and 932 of the first and second robot arms 910a and 910b. 
The conventional robot apparatus 900, however, encounters such a problem that the second and third links 962 and 963 respectively tends to interfere with the first arm links 931 and 932 of the first and second robot arms 910a and 910b as the first and second driving shafts 921 and 922 are respectively pivoted clockwise and counterclockwise around the pivotal axis. This leads to the fact that the pivotal movement range is limited in response to the outside diameters of the second link 962, the third link 963 and the first arm links 931 and 932 of the first and second robot arm 910a and 910, the distance between the central line of the second link 962 and the central line of the first arm link 931 of the first robot arm 910a, and the distance between the central line of the third link 963 and the central line of the first arm link 932 of the second robot arm 910a. 